This invention relates in general to liquid-metal-cooled nuclear reactors and more particularly to an improvement in such a reactor whereby the pressure differential between the inside and outside of core assembly ducts is reduced. The invention also relates to a modified core assembly upper adapter capable of obtaining this result and to the combination therewith of a modified instrument tree.
A nuclear reactor includes a reactor vessel into which a heat transfer fluid, typically liquid sodium for fast breeder reactors, is pumped under pressure. The fluid flows through the core and is heated; the hot fluid emerges from the vessel and flows through mechanically separated primary and secondary loops to electrical power generation equipment. Within the vessel there is supporting structure for the core components which typically include fuel rod bundles or assemblies, control rod assemblies, blanket fertile material or fertile rod assemblies and removable radial shielding assemblies.
The structure within the vessel above the core provides secondary holddown of the reactor core assemblies for the contingency that gravity and hydraulic holddowns fail (during emergencies such as control rod scram), and also supports control rod drive channels and instrumentation. Also this structure may be designed to promote mixing of cooling fluid emerging from different core components which may operate at different temperatures. This structure above the core may be called upper-core support structure, upper internal structure, upper internals or an instrument tree. All of these terms are conventional in the art. In this application the term instrument tree will be used.
Typically, in a fast breeder reactor, a substantial pressure difference exists between the inside and the outside of the ducts enclosing the core assemblies. Accordingly core assembly duct walls of substantial thickness are required. Reasons for reducing the duct wall thickness include:
The decreased mass of structural material increases the fuel volume fraction and may decrease the reactor diameter.
The decreased mass of structural material also decreases the parasitic neutron absorptions and thus improves breeding performance.
Hexagonal ducts with heavy walls are more costly than those with thinner walls (within limits).
Internal pressure within the core assembly duct is determined by the fuel assembly pressure drop caused by the coolant flow velocity and no significant reduction can be counted on. If the pressure differential is to be decreased, it must be by increasing the pressure on the outside of the ducts. Clearly, increasing the bypass flow to increase the pressure between ducts would not represent a satisfactory solution of the problem since efficiency of the reactor would thereby be reduced. Other solutions to the problem have been developed but these are not compatible with conventional fuel handling equipment and procedures employing rotating plugs and push-pull vertical in-core fuel handling machines.
It is an object of the present invention to reduce the pressure differential between the inside and outside of core assembly ducts in a fast breeder reactor.
It is a more detailed object of the present invention to increase the interassembly coolant pressure in a fast breeder reactor.
It is also an object of the present invention to provide an instrument tree for a fast breeder reactor which provides back-up mechanical holddown for the core assemblies while allowing for thermal or radiation-induced growth of core components.
It is a further object of the present invention to provide an instrument tree for a fast breeder reactor which, in addition to the above, provides mixing of coolant emerging from the core assemblies.
It is a still further object of the present invention to provide a instrument tree as described which incorporates core assembly positioning elements.
A basic object of the present invention is to achieve the above objects without interfering with well-established core assembly handling methods so that conventional rotating plugs and push-pull vertical in-core fuel-handling machines can still be used for fuel assembly loading and unloading.
It is also an object of the present invention to provide metal-to-metal contact at the corners of core assemblies, leaving the center open for coolant flow.